I absolutely love memory foam mattresses, but you have to do your research and watch out for false advertising (particularly on density).
First off, you don't need a foot+ block of memory foam, its pointless and a waste. And you definitely don't want a low density MF, as you'll have no back support and it won't be as durable. Generally standard foams are used for the core, and then different densities of memory foam for the top layers as the higher the density the MF, the higher the cost all else equal, and the lower the density the softer it feels but less support and longevity you get. 4-6lb foams are best for most, although you cooouuuuld get away w/ one of the 3lb foams as a thin top most pillow layer. Too light a topper can make some people feel "stuck" in their bed though as they sink too much and get immobilized.
On the same note, because its only the upper four inches or so of memory foam that you actually feel (memory effect wise), you can go with toppers for a similar result provided your bed is flat. People often use two 2" of different density, or a 3-4" single.
And yes, memory foam does NOT breathe and it will retain heat, however, an easy fix for that in the summer is to simply use a mattress cover with a fluffy cotton top (or artificial material like coolmax) to it, as its only the few millimeters next to your skin that really have to breathe to feel cooler, and you still get the soft feeling of the memory foam underneath. Don't bother with the "ventilated" or eggshell memory foam toppers or beds, as this is marketing nonsense. Not really harmful, but they collapse with weight on them regardless, and are simply used as it allows them to pour less expensive MF into the mold for a given size and thus save money (its similar to using a lower density or they get extra height w/ less material w/ eggshells for advertising's sake).
A nice benefit of memory foam beds btw aside from having a cool clean cut square style to them, is that you don't disturb your partner either. On mine you can jump on one side of the bed and not affect the other in the least and they don't make noise obviously.
Memory foam beds also do not need a boxspring. I personally use a bed w/ slats, just make sure the slats aren't spaced far apart for proper support of the mattress.
Hope that helps.